Simple LIMIT Clause Impossible?

Question

Is the simple LIMIT clause available in other databases simply impossible in Oracle? This is driving me nuts and I'm just not finding a good answer.
For example, the query:
SELECT * FROM MYUSER ORDER BY LOGIN LIMIT 41, 20
This would return up to 20 rows starting at row 41 in the query.
How is this done in Oracle?
I've seen ROWNUM, but:
SELECT * FROM MYUSER ORDER BY LOGIN WHERE ROWNUM > 40 AND ROWNUM < 61
This won't work because ROWNUM > x is not valid and will always return nothing. Also the ORDER BY is done after the where, so you wouldn't get the right rows anyway.
You can fix the ORDER BY problem with:
SELECT * FROM MYUSER (SELECT LOGIN FROM MYUSER ORDER BY LOGIN) WHERE ROWNUM > 40 AND ROWNUM < 61
But you still have the ROWNUM > x is invalid problem.
This seems just as silly as Oracle's NULL == '' problem. But anyway...
Let me explain why I need this and maybe someone can tell me what I can do as a workaround.
I have a web application that (among many other things) has a table of users. This table can easily reach into the tens of thousands of entries.
The web application allows the administrator to page through the list of users, sorting by different criteria, so that they can locate a particular user, group of users, or just so they can generally browse things.
I attempt to keep as little stateful information as possible so that memory (and other resources) aren't wasted.
When a user clicks on the list of users, I do a query for the first 20 rows and display them, sorted by whatever criteria the user sets. The user can then hit the "next page" link and I do another query for rows 21-40 and display those. And so on, both forward and backward, with an option to go to the first 20 and the last 20.
I'm doing all this through Java and JDBC on a proprietary web server that supports Java servlets.
I could keep a result set open and that would help with page forwards (just grab the next 20 from the set). But what about paging backward? Or jumping ahead to the end? Or jumping ahead or backward several pages?
This seems like a very common web application theme. What am I missing?
Greg
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Answer 1

To answer your last question ('What am I missing?') first - you aren't missing anything.
You are correct - this is a very common question. You will find a dozen or more similar questions if you search this forum (search on ROWNUM).
Oracle assigns ROWNUM as the result set records are being retrieved. Naturally this is before the ORDER BY clause since Oracle can't order what doesn't exist.
That is why 'ROWNUM <' works (oracle quits when the desired number of rows is reached) but 'ROWNUM >' does not work (ROWNUM will never get to the desired number).
The trick is to use a nested query or multiple nested queries. Oracle 8i supports the ORDER BY clause in nested queries.
The first (innermost) query selects records and orders them.
The second query is used to wrap the first query and assign row numbers.
The third query selects the desired range of rows from the second query.
'select * from (select rownum rn, e.* from
(select * from emp order by ename desc) e) where rn >= 4'
If an ORDER BY is not needed you can eliminate the first query.
'select * from (select rownum rn, e.* from emp e) where rn >= 4'
The inner query creates the RN column based on ROWNUM and the outer query uses the RN column (just as if it existed in a table) to select records with an RN value >= 4.
Keep in mind that the performance will go down as the number of records goes up.
For best results you need have a column in the table that you can use in the WHERE clause. Unfortunately this isn't always possible if you are trying to support generic ORDER BY clauses.
One way that is often useful to try to understand why things like this 'are the way they are' is to examine how you would perform the task manually.
Suppose I place in front of you a stack of 8 1/2 X 11 paper whose pages are not numbered and ask you to find and make copies of pages 40 - 60.
How would you do it? You would start counting from the beginning. When you reach page 40 you would copy the next 20 pages.
When you are done the pile of paper is in exactly the same state as when you started.
Now I ask you to give me copies of pages 61 to 80.
You have to start all over at the beginning as if the first request never happened.
That's pretty much what the computer has to do.
If the pages are numbered you can go directly to the desired pages.
Now what if I ask you to copy pages 20 to 30 after you sort them in order by LAST NAME?
There are exceptions (e.g. parallel processing, neural networks) but generally the computer can only do what you can do yourself. It can just do it faster and more efficiently.
Certain types of problems are not well-suited for the computer and some problems cannot be solved at all.
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